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BQ27426: Routing tradeoffs

Vasyl Vorobei
Intellectual 470 points
Part Number: BQ27426

Hi.

The BQ27426 comes in a tiny package, which requires 3/3mil tolerances to route it.

Currently, I'm working on a design with this gauge. The design consists of a relatively large (97*137 mm) 4L PCB, which fits in 6/6 mil tolerance everywhere except this fuel gauge.

I wonder if it's possible to route VSS (ball B2) through the GPOUT pin (ball A1) if we don't use the GPOUT functions. In such a manner:

Other ways exist, like an additional substrate or negotiations with the PCB house about the price. But I'd like to know every option I have.

Regards.

  • 0
    TI__Guru 61856 points

    The official TI recommendation is to connect GPOUT to a GPIO on the uC because despite its name, this pin also serves as an input during shutdown to wake up the gauge.

    If you connect it to GND permanently and you issue the shut down command, you will have to physically disconnect and reconnect the gauge from/to its power supply to start it back up again, which is often an unacceptable risk that could render the gauge on the end user product unoperational.

  • 0 in reply to Dominik Hartl11
    Intellectual 470 points

    It's clear that without GPOUT, it is impossible to use shutdown mode. Let's assume the design doesn't require shutdown mode, and sleep mode (or even a normal one) is enough. Will other limitations exist, like a logical "1" on GPOUT after POR, which might damage the IO internals?

  • 0 in reply to Vasyl Vorobei
    TI__Guru 61856 points

    GPOUT is implemented as a generic GPIO, which is initialized as an input by hardware. In addition, the gauge FW actually configures it specifically as an input in the first few lines of code so it won't drive a logic 1 after POR. I do want to emphasize the shutdown issue. This emphasis is based on actual end applications where it was a massive problem having the gauge inoperable without a good way to start it back up because GPOUT wasn't toggle-able. If you do what you intend, you must be aware that it's possible to have the gauge shut down and rendered inoperable with software only and not recoverable unless you physically disconnect and reconnect the gauge. 

  • 0 in reply to Dominik Hartl11
    Intellectual 470 points

    The possible issue with the IC shutdown lock is clear. I believe an additional substrate would be much better than the software-bricked BGA IC. Thanks for the clarification.